Line-of-sight (LOS) effects in strong lensing — weak distortions to an image induced by the presence of objects such as dark matter haloes along the LOS — have long been considered a nuisance effect when it comes to analysing strong lensing images. However, it was recently proposed that LOS shear could become a cosmological observable in its own right, if it could be accurately obtained from a given image. After recapping gravitational lensing theory, in this talk I will show how, using a new formalism, the LOS shear can indeed be measured with percent level accuracy from simulated strong lensing images, paving the way for its development as a new cosmological probe. Based on 2210.07210.